Extracting power from fluid flow is a prominent source of renewable energy. Mainstream examples include wind power and hydropower.
Traditional systems for extracting power from fluid flow are primarily turbine-based. In a turbine, one or more blades are rotatable about a central point, which is rigidly attached to an anchor (typically a tower). The blades are placed within the flowing fluid, which induces a rotation of the blades, and the rotation is converted to electricity.
Turbines may suffer from a number of drawbacks. For example, the forces exerted on a turbine are proportional to the cube of the length of the turbine blades. As the turbine blades increase in size, destructive forces (the moment about a tower, for example) are cubed. By contrast, usable power is only squared.
This “square-cube” law places significant restrictions on the scale of turbines. Inevitably, the gain of additional power extracted from greater size is not offset by the cost of addressing an increase in destructive forces. For at least this reason, turbine scale is limited.
Other known solutions eliminate towers or other rigid anchors. Examples of such power extraction systems include airborne wind energy systems (“AWE”). Typically, these systems are aerodynamic bodies tethered to the ground (a kite, for example) which fly at altitudes above the height of wind turbines.
There are two main mechanisms for extracting power from an AWE's movement through air: on-board power generation and ground-based power generation. An example of the former includes a turbine on the kite which generates electricity in the same way as the turbines discussed above. An example of the latter includes a long tether attached to a drum, where movement of the kite unrolls the tether from the drum, which rotates the drum and a connected generator, thus converting wind power into electricity.
AWEs may also suffer from a number of drawbacks. For example, because the system requires a tether angled to the airborne object, the power extracted will be a function of the available power and the cosine of the tether angle. Thus, the power extracted may never equal the available power. In addition, the tether will create drag as it moves through the air, slowing the kite, and thus reducing the harvested power. Finally, high-flying AWEs are subject to aviation restrictions, which limit their geographic scope (due to no-fly zones, for example) and present regulatory hurdles for implementation.